<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>垂中</title>
    <style>
        body{
            background-color:silver;
        }
        .box1{
            width: 400px;
            height: 200px;
            background-color: white;
            position: absolute;
            top:50%;
            left: 50%;
            margin-left: -200px;
            margin-top: -1  00px;
        }
    </style>
</head>

<body>
    <div class="box1">垂直居中,告示:......................</div>
</body>

</html>